Conventional digital optical disk generally records data by converting information into the binary data corresponding to two different physical states of record marks on a storage medium. Such storage method is referred as binary storage method. The current storage technology of read-only optical disk usually employs the binary storage method. A pit or land appeared at a record region on the disk is determined by the intensity of reflective light being low or high. Two states can be recorded in each record cell that corresponds to one bit information.
The multilevel storage technology is developed on the basis of the binary storage technology. If a data stream is modulated into M-ary digits (M>2) which correspond to M kinds of different physical states respectively, the M-level storage will be obtained. In the M-level storage method, log2M bits data can be stored in one record mark (unit). As a result, if M is bigger than 2, one record unit will record the information over one bit and the data transfer rate will be improved at the same time. The multilevel storage technology is a new technology that can prominently improve the storage capacity and data transfer rate without changing the laser wavelength and the optical numerical aperture. Therefore, the multilevel storage system has a good compatibility with the conventional optical storage systems.
An example of basic multilevel read-only optical disk is the solution of Pit-Depth Modulation (PDM). According to the scalar diffraction theory, for read-only disk, the intensity of reflective light corresponds to the depth of record pit on a disk: the intensity of reflective light decreases along with the increment of pit-depth at the beginning of zero, and when pit-depth is a quarter of laser wavelength, the intensity of reflective light becomes minimal value. The multilevel optical storage method can be achieved by setting the different pit-depth considering the relation between pit-depth and the intensity of reflective light. But numerical pit-depth levels may cause many technical problems, such as disk replication and signal detection. Therefore, the improvement of the storage capacity of the read-only disk by only increasing the levels of pit-depth is limited severely.
Conventional modulation coding of multilevel optical storage generally employs the solution of amplitude modulation which does not dig up the potential of the coding technology thoroughly. However, all of the conventional binary storage disk use the run length limited (RLL) modulation coding. RLL means the sequence in the optical storage channel must be constrained: such a constrained sequence must have a least d ‘zeros’, but at most k ‘zeros’ between two ‘one’ in the sequence. Parameters d and k are the minimal and maximal values of run length in data sequence. Because d limits the max value of data transfer rate, it can influence the inter symbol interference (ISI) of the sequence throughout a band limited channel. In the transfer of binary data, signals received are usually expected to be self-synchronizing. The synchronization is achieved by a phase lock loop for regulating the phase at a detecting time in response to the waveform jump received. The maximal run length parameter k ensures a fit jump frequency so as to meet the requirement of reading clock synchronization.
RLL codes can increase the storage capacity relative to the amplitude modulation codes. In the binary storage method using the RLL codes, information over 1 bit can be recorded in a minimal record mark, so RLL codes are widely used in the optical storage technology, such as the EFM code (d=2, k=10) for CD and the EFM+ code (d=2, k=10) for DVD. Since the RLL code is used, the recording density of DVD reaches 1.5 bit per minimums symbol.
In view of the foregoing, it is desired to provide a multilevel read-only optical disk and method for producing the same.